Pretreatment of a metal surface for conversion coating



United States Patent Ofitice 3,370,992 Patented Feb. 27, 1968 3,370,992PRETREATMENT OF A METAL SURFACE FOR CONVERSION COATING Frank P. Ilenda,Mentor, and Bert E. Palm, Mentor-onthe-Lake, Ohio, assignors to DiamondShamrock Corporation, a corporation of Delaware No Drawing. Filed Oct.14, 1963, Ser. No. 316,182 17 Claims. (Cl. 148-616) ABSTRACT OF THEDISCLOSURE A metal surface is treated in a bath containing phosphoricacid and a chlorate maintained at a pH of less than 2.7. Surfacestreated in this manner are more re ceptive to a subsequently appliedcorrosion-resistant and siccative paint bonding coat. Finally, to thisbonding coat there is applied an organic finish.

The present invention relates to a composition and process for thetreatment of metal surfaces and more particularly relates to acomposition for and a process of preparing or conditioning metalsurfaces to recive a subsequent protective coating.

It is well known that bonding coatings are desirable on metallicsurfaces to protect such surfaces from corrosion and to form a base topromote adherence to such surfaces of organic finishes, generally in theform of paints, enamels, lacquers, etc. In the commercial manufaoturingof metallic sheets, strips or the like, it has become customary tosubject the sheet or strip to a treatment usually at the mill to protectthe surface and prevent the formation of corrosion products in theinterim between production of the sheet and the ultimate finishing ofits surface and its intended end use. A variety of processes are usedfor the purpose and many specifically different compositions have beenproposed as satisfactory for use in such processes.

At the present time well over 90 percent of all bonding coats on metal"are phosphate coats or chromic acid in combination with any of theother activator ingredients such as chlorides, sulfates, fluorides,phosphates, complex fluorides, borates, etc. to form a protectivecoating on the surface. They generally are applied in the followingmanner: the sheet metal or strip is first thoroughly cleaned in analkaline solution, the adherent solution then being removed by a waterrinse. The clean metal is next either immersed in a hot bath or sprayedwith a solution containing the bonding coat material until a coating ofdesired thickness or weight is formed. Generally, the weight of the bondcoating is in the range of about 30 to 200 or more milligrams per squarefoot, preferably about 50 to 150 milligrams per square foot. A waterrinse follows to remove all coating solution and nonadherent sludge.Some of these processes produce colorless coatings while others producegray-colored coatings, but all are considered to contain a metallicphosphate or chromate or their combination as one of the coatingcomponents. After the water rinse, the bonding coat is dried at anyconvenient temperature as by air drying, since there is nothing requiredbeyond the removal of the moisture from the bonded surface.

When an attempt is made to apply an organic finish to the bonding coat,it has been found that the use of conventional cleaning or conditioningprocedures prior to the application of the siccative pain't bonding coatare ineffective to promote the desired degree of adherence of organicfinishes to the bonding coat on the metallic surfaces. For example, theorganic finishes are relatively easily chipped, flaked or peeled fromthe metal surface, when the metal is subjected to bending or mechanicalshock, and such removal of the finish leaves portions of the surfaceexposed to sources of corrosion which may be in contact therewith.

In accordance with the present invention, it has now been found thatmetal surfaces pretreated with an oxidizing bath containing phosphoricacid and a chlorate as the sole oxidizing agent, while maintaining theacidity of the bath at a pH level of less than 2.7, are rendered muchmore receptive to an adherent cororsion-resistant and siccat-ivepaint-bonding coat than has previously been obtained by prior artmethods. The pretreatment process of the present invention is of specialimportance where the si-ccative paints are applied to metal articles andthe articles "are thereafter exposed to bending stress which wouldordinarily cause the organic finish, usually containing color or lacquercoatings, to crack or peel were it not for the pretreatment on the metalsurface.

The use of a solution containing a phosphorus-containing compound and anoxidizing agent such as a chlorate has been previously proposed for thetreatment of metal surfaces. However, the use of suchphosphoruscontaining solutions has been for the purpose of produc ing apassivating phosphorus-containing coating on the metal surface.Generally, the phosphorus-containing compound is added to the solutionin the form of either an alkali metal phosphate or a Group II metalphosphate to form a substantially insoluble rust and corrosionqesistan't phosphate coating on such surfaces. Thus, conventionalphosphate coatings are formed by a process using a single bath whichcontains phosphates, an oxidizing agent and sometimes other ingredients.The metallic article to be coated is placed in such bath and therequired reactive conditions are established and maintained until thedesire-d coating is formed.

As distinguished from such conventional processes, the present inventionutilizes a novel solution for, and a process of, preparing orconditioning metal surfaces to receive a bonding coat. In accordancewith the present invention the properly cleaned metallic articles to betreated are immersed in or sprayed with an oxidizing bath containingabout 0.5 to '10 percent, preferably about 1 to 3 percent, phosphoricacid by weight and 1 to 10 .grams, preferably 1 to 5 grams, of chlorateion per liter of bath solution and the balance water while maintainingthe acidity of the bath at a pH level within the range of from 0.5 to2.7, preferably 1 to 2.2 at a bath temperature of between about 50 to212 F., preferably to 180 F.

The time of reaction during which the articles to be treated aresubjected to the action of the solution depends on the composition ofthe metal, the nature of the surface, and the purpose for which thetreated working piece is intended to serve. As a general rule, thearticles need only be immersed in or sprayed with the solution for ashort time, for the reaction taking place at the surface of the metal tobe completed. Generally, it is sufficient when operating at temperaturesof between 50 and 212 F. to treat the surface for a period of time up toabout 3 minutes but generally varying from about 5 seconds to 5 minutesand preferably about 15 seconds to 2 minutes. As a general rule, anincrease of the temperature does not require a substantial alteration ofthe treatment period, but it has been found that by increasing thetemperature, the time during which the article to be treated issubjected to the action of the solution may be decreased. F or instance,at temperatures of 70 B, it has been found that the pretreatment iseffected after a period of about 2 minutes; at 140 F. after a period of1 minute; and at 180 F. after a period of 15 seconds.

Before starting the treatment of the present invention it is, in mostcases, advisable to remove foreign matter from the metal surfaces bythoroughly cleaning and degreasing. The de-greasing may be accomplishedwith known agents, for instance, with agents containing sodiuminetasilicate, caustic soda, carbon tetrachloride, trichlorethylene,etc.

It is to be understood that the term phosphoric acid is intended toinclude meta-phosphoric, ortho-phosphoric, pyro-phosphoric,trip0ly-phosphoric, tetra-poly-phosphoric acid and other polyphosphoricacids. The chlorate ions may be conveniently introduced into the bathsolution in the form of a metallic salt and preferably as the sodiumsalt. However, other metal salts such as potassium, barium, or magnesiumchlorates may also be used.

When repeatedly used, the aqueous solution shows a certain depletion ofits ingredients since they have been consumed in the treating of themetal surface. For this reason, the ingredients of the solution arereplenished at a rate corresponding to their consumption, by introducingsolutions containing the desired ingredients to the original solution tomaintain the pH level within a range of about 0.5 to 2.7. Also, to avoidthe undesirable formation of sludge in the treating bath, and to allowcontinual operation, it has been found advantageous to add about 1 tograms per liter of solution, preferably 1 to 5 grams per liter ofsolution, nitric acid.

In the preferred practice of the present invention, the metal surfacesmay be after-treated by thoroughly rinsing them with water in order toremove adhering ingredients of the bath, and thereafter Withoutpermitting the article to be dried immersing the pretreated metalarticle into the coating bath described hereinbelow.

The siccative paint-bonding coat which may be applied to the pretreatedmetallic surface can include any of the bond coatings commercially used.For instance, Table I contains various formulations ofchromate-phosphate solutions which may be used in forming the siccativepaint-bonding coating.

TABLE I Grams/liter Chromic acid (CrO 40-200 Magnesium oxide 12-60Phosphoric acid 15-75 Ethylene glycol 7-35 Nonionic wetting agent0.2-0.3

(Equivalent amounts of ammonium bichromate may be substituted for thechromic acid.)

Grams/liter Chromic acid (CrO 40-200 Calcium carbonate -100 Phosphoricacid 12-60 Ethylene glycol 7-35 Nonionic wetting agent 0.2-0.3

(Equivalent amounts of ammonium hichromate may be substituted for thechromic acid.)

P-octyl phenyl tri-oxyethylene ethanol. 0.05 Water, to make 4 liters. X

Parts Chromic acid (CrO 3 Triethanolamine 1 Water 96 Chromic acid (CrO3' Acrysol A-1 (polyacrylic acid) 3 Water 94 XII Chromic acid (CrO 4Zinc oxide 1.42 Sucrose 1.3 Water 93.28

III

' Grams/liter Chromic acid 40-200 Calcium carbonate 2 1-5 Phosphoricacid 12-60 Zinc oxide 14-69 Ethylene glycol 7-35 Nonionic wetting agent0.2-0.3

1 Ammonium bichromate may be substituted for the chromic acid.

2 Calcium oxide may be substituted in equivalent amounts for the calciumcarbonate.

Aluminum hydroxide, magnesium oxide or magnesium carbonate may besubstituted in equivalent amounts for the zlnc oxide.

Water, to make 1 liter.

An alkyl aryl polyether alcohol with 10 ethylene oxide groups.

Parts Chromic acid (CrO 0.90 Phosphoric acid 0.85 Ethylene glycol 0.25Water 98.00

VII

Ammonium bichromate 2.0 Ammonium dihydrogen phosphate 1.0 Hypophosphorusacid 0.5 Water 96.5

VIII

Zinc dichromate 2.8 Dow Latex 566 (butadiene-styrene copolymerdispersion-the percentage being based on copolymer solid Weight) 2.0Sucrose 0.6 Triton 102 1 0.04

Water 94.56

An alkyl aryl polyether alcohol made by condensing one mol ofp-(n-octyl)phenol with three mols of ethylene oxide.

Grams Zinc dichromate 6 Sucrose 2 Aqueous dispersion ofpolytetrafluoroethylene having 50% resin solids content by weight, 4 cc.

The bond-coating solution may be applied to the pretreated metal surfaceeither by spraying or by dipping the article into the soltuion. Themetal carrying film of coating solution may be heated to a temperatureof from about 100 F. to about 400 F., at which temperature the oxidantsand reducing agents are substantially completely destroyed in thecoating film with reduction of hexavalent chromium and formation of ahighly adherent, oxidation-resisting, chemically inert, bonding coatconsisting chiefly of compounds of chromium and phosphorus, probablychromic phosphate.

The bonding coat which forms on the pretreated metal surface iscontinuous, thin, highly adherent, hard, smooth, and lustrous, andwithout nonadherent material. These features permit repeated flexing ofthe coated metal through 360 degrees, deep-drawing of the metal up tothe point where the metal itself tears, and similar fabricatingoperations involving deforming techniques, without destruction of thecoating. Thus, organic finishes when applied to siccative paint-bondingcoatings on metal surfaces which have been pretreated according to thepresent invention, do not decompose at the interface and are verydiificult if not impossible to remove from the pretreated metal surfacecontaining the bonding coat, short of destruction of the metal surfaceitself. Thus, metal surfaces pretreated according to the method of thepresent invention may receive organic finishes having an end use as:automobile finishes, can coatings, appliance coatings, precoatings ofstrip metals, coatings on metal building panels (house siding, gutters,downspouts, etc.), mobile home exteriors, exterior trim, metal cabinetsand tables, and the like.

Exemplary of organic finishes which may be applied to the bond coatinginclude: epoxy resins, e.g. reaction product of epichlorohydrin andpolyhydric phenols, oxidation of olefins with peracetic acid orphenol-formaldehyde condensate esterfied with epichilorohydrin vinylresins (homopolymers and copolymers), e.g. polyvinyl chloride, polyvinylfluoride, polyvinyl acetate, polyvinyl butyral, etc.; acrylic esterresins, e.g. polymerized esters of acrylic and a-methacrylic acid and acopolymer of methyl acrylate and hydroxyethyl met-hacrylate;cellulose-based finishes, e.g. cellulose acetate, nitrocellulose andcellulose acetate ,butyrate; polyester resins, e.g. tetrahydrophthalicanhydride based polyesters, etc.; polyurethanes, e.g. reaction productof hydroxyl bearing resins and a diisocyanate; silicones; rubber resins;alkyds, e.g. reaction product of polybasic acids and polyhydricalcohols; amino resins, e.g. urea-formaldehyde and melamineformaldehyde;and phenolic baking resins, e.g. phenolformaldehyde resins. Organicfinishes may be applied to the bonding coating up to a thickness ofabout 100 to 200 mils when used in corrosion type coatings; but,generally, they are applied in the range of about 0.2 to 20 mils. Afterapplying the organic finish to the bond coating, the coated article isheated for a suificient period of time to achieve essentially a dryorganic finish. Temperatures in the range of about 70 to 600 F. aregenerally used for a period of time from a few minutes up to about 48hours, usually from about 1 hour to about 24 hours.

Characteristics of the above-mentioned organic finishes and applicationmethods for applying these finishes to surfaces are discussed more fullyin the following publications; American Paint Journal, September 1963,p. 112A; Aznerican Finishing Guidebook, Directory for 1963, PlasticsPublication, Inc. p. 537; and Finishing Handbook & Directory 1963,Product Finishing, Sawell Publications Ltd., London, p. 129, herebyincorporated by reference.

Metal surfaces which may be treated according to the practice of thepresent invention include the ferrous metals, zinc, aluminum, and theiralloys.

In order that those skilled in the art may more completely understandthis invention and the methods by which the same may be carried intoeffect, the following specific examples are offered:

Example 1 TABLE II Sample HaPO4, N 21010 NH4H2PO4, Estimated, Actual,

ce./l. g.l/. g. /l. pH p 3 a t it 6 3% 2% 3 7% 3 2. 7

1 1 gram of NaOH is also added.

Seven steel panels (4" x 12") are cleaned with an aqueous solutioncontaining g./l. NaOH and 10 g./l. KMnO Into each of theabove-identified pretreatment bath solutions is dipped one of the sixpreviously cleaned steel panels for 45 seconds at a temperature of 140F. The steel panels are then removed from the bath solutions and rinsedwith water. The water-rinsed pretreated steel panels are then dippedinto a 10% bond-coating solution consisting of:

Grams Cr0 a 400 ZnO 138.6 H PO (100%) 120 caco 10 Ethylene glycol 74.4

Water, diluted to 1 liter.

The painted steel panels are subjected to a bend test conducted inaccordance with the standard method of ASTM B-522-41, which generallyinvolves bending a lengthwise portion of the panel through 180 around aconical mandrel (Gardener Laboratories Conical Mandrel), having a smalldiameter of A inch, the diameter four inches from the small end beingone inch, and a length of 8 inches. The results of the bending test arereported in terms of flaking or fracture (craze) of the paint system.Flaking indicates separation of the coating from the panel surface.After the steel panels are subjected to the bend test, they aresubjected to the Scotch adhesion test using Scotch filament tape #880.The results of the bend and adhesion tests are given in Table III.

TABLE III Actual pH Sample of Pretreat- Bend Test Adhesion Test mentBath No flaking or fracture- No peeling. do Do.

1 N o pretreatment of Sample G.

It can be seen from the test results that, when the pH of thepretreatment bath is at a level of 2.7 or above the paint system flakesand fractures when the painted panel is subject to the bend test;Whereas when the pH level of the pretreatment bath is below 2.7, e.g.1.8 and 2, respectively, no flaking or fracture occurs and the paintsystem is not removed by the Scotch adhesion test, indicating goodadhesion to the metal surface. Although Sample G, the steel panel notsubjected to a pretreatment bath, passes the bend test, the paintcoating is easily rem moved from the metal surface by the Scotchfilament tape.

Example 2 Three 4" X 12" steel panels (Samples H to L) are cleaned withan aqueous solution containing 20 g./l. r NaOH and g./l. KMnO Each ofthese steel panels are then dipped into an aqueous pretreatment bathsolution for 1 minute at a temperature of 140 F. The pretreatment bathcontains 10 cc./l. of H PO (85%) and 3 grams/l. NaClO and has a pH of1.8. The pretreated steel panels are removed from the pretreatment bathsolution and rinsed with water. Each panel is then dipped in abond-coating solution, removed and heated for 3 minutes at 350 F. Thebond-coating solutions are identified in Table IV below.

The three steel panels are then painted with an acrylic paint system andbaked at 385 F. for 20 minutes. The painted steel panels are thensubjected to the bend and adhesion tests described in Example 1. Theseresults are also reported in Table IV.

Sample K, Coating Bath:

Chromic Acid N o flaking or frac- No peeling.

(CrOz), 3%. ture. Acrysol A-l (polyacrylic acid) 3%.

Water, 94%

1 Sufiicient for reducing of chromium from hexavalcnt state to trivalentstate upon heating to 350 F.

2 Suflicient for reducing 60% of chromium from hexavalent state to 50trivalent state upon heating to 350 F.

Suflicient for reducing of chromium from hexavalcnt state to rivalentstate upon heating to 350 F.

S It can be seen from the above results, when chromic acid is the solecoating material in the bond coating bath, it is necessary to reduce100% of the chromium from the hexavalent state to the trivalent state toachieve good paint adhesion to the pretreated metal surfaces.

Example 3 Samples M to R, (tour 4" X 12" panels of each) are preparedusing various treating and bond coating compositions. Samples P to R arecommercially prepared metal panels containing zinc phosphate as the bondcoating. TWO panels of each sample are painted with an epoxy paintsystem and the remaining two panels of each sample are painted. With anacrylic paint system. The treatment of the metal panels are described inTable V, below.

One panel of each sample, painted With the epoxy paint system; and onepanel of each sample, painted with the acrylic paint system, are subjectto an appliance detergent test which comprises immersing the paintedsamples in a 1% detergent solution, maintained at a temperature of 165Hi3. The detergent solution contains:

Grams IJQ4P207 N a SO 19 Naccanol NRSF 1 2O Nazslos 7 NHZCOS 1 Water, to1 liter.

A surface active agent-sodium alkyl aryl sulfonate.

The remaining painted panels are subjected to the Gravelometer testwhich indicates adhesion of the paint system to the metal surface andcomprises shooting 20 pounds of steel shot with 60 pounds of airpressure against the painted surfaces. The results of the appliancedetergent test and the Gravelometer test are also reported in Table V.

As can be seen from the Table V results, Samples M and N, pretreatedwith the composition for conditioning metal surfaces, are equivalent tocommercially prepared coated metal surfaces, Samples 0 to R.

Other painted panels treated with a phosphoric-acid chlorate bath (pH1.8), prior to the application of the bonding coat, were subjected tothe standard humidity test and after hours the paint on the panelsblistered. This test indicates good adhesion of the paint system to thebond coating.

TABLE V Detergent Test Gravclometer Test Sample M (Steel):

I. 4 cleaned steel panels are pretreated with phosphoric acid-chloratebath (pH 1.8) for 45 seconds at a temperature of 14 F. II. Rinsed withwater and dipped into the 10% bond coating bath of Ex. 1. III. Bondcoated panels are heated for 3 minutes at 350 F. IV. Two of the steelpanels are coated with an acrylic paint and the other two panels arecoated with an epoxy paint. V. The painted steel panels are baked for 20minutes at 385 I Sample N (Galvanized Steel):

and epoxy) observed.

I.V. 4 cleaned galvanized steel panels are treated the same as .do Do.

Sample M. Sample 0 (Galvanized Steel):

I. 4 cleaned galvanized steel panels are pretreated with a bath solu-..do No chipping of paint system (acrylic tion containing 20 g./l. NaOHand 10 g/l. KMnO, for 30 seconds and epoxy) after 600 hours. at atemperature of F. IL-V. Same treatment as Sample M. Sample P (GalvanizedSteel): I

1. Previously prepared zinc phosphate bond coating on gal- ....do N ochipping of paint system (acrylic vanized steel is treated according toSteps IV and V of Sample M. or epoxy) observed. Sample Q (Steel):

I. Previously prepared zinc phosphate bond coating on steel 15 .do D

treated according to Steps IV and V of Sample M. Sample R (GalvanizedSteel) 1. Previously prepared zinc phoshpate bond coating on a particud0Dc.

laigalvanized steel is treated according to Steps IV and V ol Sample M.

1 Supplied by Weirton Steel Co.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited, since changes and alterations therein may be made whichare withinthe full intended scope of this invention as defined by theappended claims.

It is claimed:

1. A method for conditioning the surface of a metal selected from thegroup consisting of ferrous metal, Zinc, aluminum, and alloys thereof toreceive a bonding coating selected from the group consisting ofchromate, phosphate and chromate-phosphate bonding coats which methodcomprises the steps of applying to said surface an aqueous solutionmaintained at a pH level less than 2.7, said solution consistingessentially of phosphoric acid and a metal chlorate as an oxidizingagent, said phosphoric acid being present in said solution from 0.5 to10 percent by weight, and from 1 to 10 grams of chlorate ion per literof solution.

2. The method of claim 1 wherein the aqueous solution is maintained at apH from 1 to 2.2.

3. The method of claim 1 wherein said metal chlorate is selected fromthe group consisting of sodium, potassium, barium and magnesiumchlorate.

4. The method of claim 1 wherein the phosphoric acid is present in anamount from 1 to 3 percent by weight of solution.

5. The method of claim 1 wherein the metal chlorate is sodium chlorateand the chlorate ion is present in the aqueous solution in an amountfrom 1 to 5 grams per liter of solution.

6. The method of claim 1 wherein the temperature of the aqueous solutionis within the range of 50 to 212 F. and the contact time is for a periodof time up to 30 minutes.

7. The method of claim 6 wherein the temperature of the aqueous solutionis within the range of 70 to 180 F., and the contact time is from 10seconds to 5 minutes.

8. The product produced by the method of claim 1.

9. A method of conditioning the surface of a metal selected from a groupconsisting of ferrous metal, zinc, aluminum, and alloys thereof, toreceive a bonding coating selected from the group consisting ofchromate, phosphate and chromate-phosphate bonding coats which methodcomprises the steps of applying to said surface an aqueous solutionmaintained at a pH from 0.5 to 2.7 in a temperature range from 50 to 212F., and the contact time is from 10 seconds to 5 minutes, said solutionconsisting essentially of 1 to 10 percent by weight of phosphoric acidand from 1 to 5 grams per liter of solution of chlorate ion, saidchlorate ion being supplied to said solution by sodium chlorate.

10. A method for producing an adherent bonding coat selected from thegroup consisting of chromate, phosphate and chromate-phosphate bondingcoats on a metal base selected from the group consisting of ferrousmetal, zinc, aluminum and alloys thereof which method comprises thesteps of applying to said base an aqueous solution maintained at a pHlevel less than 2.7, at a temperature within the range of 50 to 212 F.for a period of time up to 30 minutes, said solution consistingessentially of phosphoric acid and a metal chlorate as an oxidizingagent, said phosphoric acid being present in said solution from 0.5 to10 percent by weight, and from 1 to 10 grams of chlorate ion per literof solution; removing the treated metal base from said solution, rinsingsaid treated metal base with water, applying to said treated metal basea bonding coat selected from the group consisting of chromate, phosphateand chromate-phosphate bonding coats having a bonding coat weight of 30to 200 milligrams per square foot and heating said applied bonding coatto a temperature suificient to substantially destroy the oxidants andreducing agents in said bonding coat.

11. The method of claim 10 wherein the applied bonding coat is heated toa temperature of 100 to 400 F.

12. The product produced by the method of claim 10.

13. A method of producing an adherent organic finish on a metal baseselected from a group consisting of ferrous metal, zinc, aluminum andalloys thereof and having a bonding coating thereon selected from thegroup consisting of chromate, phosphate and chromate-phosphate bondingcoats which method comprises the steps of applying to said base anaqueous solution maintained at a pH level less than 2.7, at atemperature within the range of 50 to 212 F. for a period of time up to30 minutes, said solution consisting essentially of phosphoric acid anda metal chlorate as an oxidizing agent, said phosphoric acid beingpresent in said solution from 0.5 to 10 percent by weight, and from 1 to10 grams of chlorate ion per liter of solution; removing the treatedmetal base from said solution, rinsing said treated metal base withwater, applying to said treated metal base a bonding coat selected fromthe group consisting of chromate, phosphate and chromate-phosphatebonding coats having a bonding coat weight of 30 to 200 milligrams persquare foot, heating said applied bonding coat to a temperaturesuflicient to substantially destroy the oxidants and reducing agents insaid bonding coat, and applying to said bonding coat an organic finish.

14. The method of claim 13 wherein the thickness of the applied organicfinish is 0.2 to 20 mils.

15; The method of claim 13 wherein the organic finish is heated to atemperature in the range of to 600 F. for a period of time suflicient toachieve an essentially dry organic finish.

16. The method of claim 13 wherein the applied bonding coat is heated toa temperature of to 400 F.

17. The product produced by the method of claim 13.

References Cited UNITED STATES PATENTS 2,318,656 5/1943 Thompson148--6.l6 2,813,815 11/1957 Miles 148-6.16 X 3,144,361 8/1964Klinghoffer 148-6.15 X

RALPH S. KENDALL, Primary Examiner. ALFRED L. LEAVITT, Examiner.

